The operations of many floating vessels in the oil and gas industry, such as semi-submersible drilling rigs, drill ships, and pipe-laying ships, are impeded by sea swell. Sea waves impart an up-and-down motion to a vessel, referred to as “heave” or “vibration,” with the period of the waves ranging anywhere from a few seconds up to about 30 seconds or so and the amplitude of the waves ranges from a few centimeters or inches up to about 15 meters (about 50 feet) or more.
This up-and-down motion imparted to the vessel is then correspondingly imparted to any loads or structures attached to the vessel. This heave motion of the loads or structures extending from the vessel is often highly undesirable, and even dangerous, to equipment and rig personnel. For example, when attempting to drill a wellbore in the seabed, the heave motion can cause a corresponding reciprocating motion of the drill string. Because one end of the drill string is coupled to the platform while the opposite end is coupled to the drill bit in the wellbore, the up-and-down movement of the drill string can vary the weight on bit (WOB) and this can adversely affect the drilling operation.
Heave compensation is directed to reducing the effect of this up-and-down motion on a load (e.g., the drilling string) attached to the vessel. Heave compensation systems may be used that typically involve measuring the movement of the vessel using a measuring device, such as a motion reference unit (“MRU”), and using a signal from the MRU that represents the motion of the vessel to compensate for the motion. The signal is used to control a motion compensation system that substantially cancels out the heave or vibration of the floating vessel due to the ocean waves. The principle behind heave compensation is to control the motion compensation system in a manner equal to but opposite the heave motion of the vessel to cancel out heave so the desired motion of the load is achieved irrespective of the motion of the vessel.
Presently, the signal to control conventional motion compensation systems is provided using traditional proportional-integral-derivative (PID) controllers. These PID controllers generate the signal by reacting to an error value between a measured process variable at a given instance and a desired setpoint. As a result, the PID controllers constantly react to the error and wellbore operations are not performed in a steady and controllable manner.